Light olefins, defined herein as ethylene, propylene, and mixtures thereof, serve as feedstocks for the production of numerous important chemicals and polymers. Heavy olefins are defined herein as the hydrocarbon products heavier than light olefins. Light olefins traditionally are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies, based on alternative feedstocks have increased.
Important types of alternate feedstocks for the production of light olefins are oxygenates, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The reaction, which converts oxygenates to desired light olefins, also produces by-products. Representative by-products include, for example, alkanes (methane, ethane, propane, and larger), C4+ olefins, aromatic compounds, carbon oxides and carbonaceous deposits (also referred to as “coke”).
During the conversion of oxygenates to light olefins, carbonaceous deposits accumulate on the catalysts used to promote the conversion reaction. As the amount of these carbonaceous deposits increases, the catalyst begins to lose activity and, consequently, less of the feedstock is converted to the light olefin products. At some point, the build up of these carbonaceous deposits causes the catalyst to reduce its capability to convert the oxygenates to light olefins. When a catalyst can no longer convert oxygenates to olefin products, the catalyst is considered to be deactivated. Once a catalyst becomes deactivated, it must be removed from the reaction vessel and replaced with fresh catalyst. Such complete replacement of the deactivated catalyst is expensive and time consuming. To reduce catalyst costs, the carbonaceous deposits are periodically fully or partially removed from the deactivated and/or partially deactivated catalyst to allow for reuse of the catalyst. Removal of the deactivated catalyst and/or partially deactivated catalyst from the reaction process stream to remove the carbonaceous deposits is typically referred to as regeneration and is typically conducted in a unit called a regenerator.
Previously in the art, catalyst regeneration was accomplished by removing the deactivated catalyst from the process stream, removing the carbonaceous deposits from the catalyst, and then returning the regenerated catalyst to the reactor near the inlet of the reactor or reaction vessel. Conventionally, this inlet is located near the bottom quarter of the reactor or reaction vessel. By returning the regenerated catalyst near the inlet of the reactor, the regenerated catalyst would immediately contact fresh feedstock and begin conversion of the feedstock. However, doing so does nothing to control the conversion of the feedstock into by-products.
For example, U.S. Pat. No. 4,873,390 to Lewis et al. teaches a process for catalytically converting a feedstock into a product in which the feedstock is contacted with a partially regenerated catalyst. Lewis et al. describes that a partially regenerated catalyst improves the selectivity of the process to the light olefin products. While contacting the feedstock with a partially regenerated catalyst may improve the selectivity of the process to the light olefin products, it does nothing to control production of by-products.
For these reasons, a need exists in the art for improved processes which increase light olefin selectivity and control production of by-products.